1. Field of the Invention
The present invention relates to a control system for controlling currents in an electronic system. More particularly, the present invention relates to a control system for controlling the current in drivers for solid-state lighting (SSL) devices, such as, for example, a light emitting diode (LED) or an array or cluster of light emitting diodes (LEDs) having driver circuitry that is compatible with electronic transformers and supports dimming functionality. It also improves the compatibility of LED systems to certain types of electronic transformers by enhancing current flow through the transformer.
2. Background of the Invention
Solid-state lighting (SSL) refers to a type of lighting that utilizes light-emitting diodes (LEDs) as sources of illumination rather than electrical filaments or gas. There are several types of LEDs popularly used in solid-state lighting (SSL). The first type of LED, and the most commonly used type of LED, is the semiconductor LED. A semiconductor LED consists of a chip of semiconducting material impregnated or doped with impurities to create a p-n junction. Other types of LEDs suitable for SSL include organic LEDs or light emitting polymers (LEPs). In these LEDs, an emissive electroluminescent layer is composed of a film of organic compounds. The layer usually contains a polymer substance that allows suitable organic compounds to be deposited which are capable of producing light. As used herein, the term, “light emitting diode (LED)” includes any type of such solid state lighting device including semiconductor LEDs, organic LEDs or LEPs.
The benefits and wide-range applicability of LEDs in today's lighting systems are now realized and recognized by those skilled in the art. For many years, halogen-based lamps were the primary light source implemented within lighting systems. Over the past years as LED technology has developed, the advantages of LEDs over halogen lamps have become increasingly apparent. When compared to halogen lamps, LEDs are relatively smaller, and have a longer operating life. Another important difference between halogen bulbs and LEDs is the significantly less amount of power required by LEDs to operate. For example, a halogen lamp may operate within a range of 20-50 Watts and an LED at about 5-15 Watts.
The term “solid-state” refers to the fact that light in an LED is emitted from a solid object (e.g., a block of semiconductor or organic layers or materials) rather than from a vacuum or gas tube, as is the case in traditional incandescent light bulbs and fluorescent lamps. Unlike traditional lighting, SSL creates visible light with reduced heat generation or parasitic energy dissipation. In addition, its solid-state nature provides for greater resistance to shock, vibration, and wear, thereby increasing its lifespan significantly. The watts-per-lumen output of SSLs are also higher than incandescent light bulbs and fluorescent lamps. These advantages make SSLs particularly attractive for commercial and domestic lighting purposes, and are, therefore, increasingly replacing filament or gas-based lighting applications.
When LEDs are used for lighting applications, a cluster or an array of LEDs is used to achieve the requisite brightness and other desired lighting characteristics. Regardless of color, type, color, size or power, all LEDs work the best when driven with a constant current. LED manufacturers specify the characteristics (such as lumens, beam pattern, color) of their devices at a specified current value. One or more LED drivers are used to effectively control the electrical characteristics of the array of LEDs to suit the lighting. A LED driver is a self-contained power supply that has outputs matched to the electrical characteristics of the array of LEDs. Most LED drivers are designed to provide constant currents to operate the array of LEDs.
SSLs are powered in the same way as other lighting applications, namely, starting with and using an alternating current (AC) power source. Depending on the geographic location or application, the AC source could range between 110V and 240V. The frequency of these AC sources ranges between 50 Hertz and 60 Hertz. When AC power sources are used in SSLs, there is a need for power factor correction (PFC) to minimize losses in the AC power. PFC is the ratio between the actual load power and the apparent load drawn by an electrical load, e.g., the LED driver. PFC is a measure of how effectively the current is being converted into useful work output and, more particularly, is a good indicator of the effect of the load current on the efficiency of the power supply system.
In prior art approaches, LED drivers with PFC comprise at least two popular processing stages. The first stage is the power factor correction stage, which produces a regulated high voltage. The second stage includes a DC/DC/LED driver stage, which delivers a DC current to the array of LEDs. This approach requires a high value capacitor across the array of LEDs to effectuate the load delivery. Although this approach enables PFC in SSL, it has several disadvantages. These disadvantages include the use of higher number of total components to drive the LED driver, and a corresponding increase in cost of the LED driver circuitry. This approach also includes operational inefficiencies, as it requires the input AC source to be converted to DC before applications to the array of LEDs. Prior art approaches also require the LED current to be measured and monitored in order to achieve PFC, further contributing to the operational inefficiency.
Presently MR16 lamps use electronic transformers to supply the low voltage that is needed to operate these lamps. Typically, these MR16 lamps use halogen lamps that are purely resistive and draw about 20-50 watts of power. The electronic transformers are designed to operate with these halogen lamps and require a certain minimum power to operate properly. In transitioning MR16 LED systems, it is desirable to allow these LEDs to operate without having to substitute pre-existing electronic transformers.
The electrical characteristics of the electronic transformers require a certain amount of current within the transformer. If the current through the transformer falls below a threshold, the transformer will effectively turn off until current rises above the threshold. However, the amount of power drawn by an LED is significantly less than the amount drawn by a halogen lamp. As a result, the compatibility of MR16 LEDs to pre-existing electronic transformers is a significant design issue.